Storm Forecast
Valid: Sun 02 Nov 2008 06:00 to Mon 03 Nov 2008 06:00 UTC
Issued: Sat 01 Nov 2008 17:19
Forecaster: TUSCHY

SYNOPSIS

Disturbed pattern persists as intense trough is still parked over western Europe and attendant WAA supports ridging over SE Europe. Numerous disturbances also cross far N-Europe, but limited moisture acces at LL should keep thunderstorm chances down. A few thunderstorms beneath a weak upper impulse are forecast over Romania / SW Ukraine, confined to daytime heating.

DISCUSSION

... Sardinia,Corsica and parts of the Tyrrhenian Sea ...

A baroclinic zone at all levels sets up over E/NE Algeria/N-Tunisia and the area of interest, separating hot and dry air S/E of Sicily from cool and moist air to its west. This zone will be the focus for thunderstorm initiation mainly after sunset.

Numerous, weak disturbances come out of the base of the still positive tilted upper trough and there are no signs that any of those will be well defined. Hence, we think that timing of thunderstorm onset depends on the arrival of the very moist air from the SSE, which has its origin south of Sicily where SSTs remain well above 20°C. As winds back and strengthen south of Sardinia, a very dry plume of air spreads northwards from N-Tunisia, affecting the island during the daytime hours, which should limit the potential for initiation. At the same time, a plume of very moist air south of Sicily also spreads to the NNW, finally arriving during the evening hours over Sardinia. At the same time, another weak upper impulse draws near from the SW, so environment should indeed become favorable for initiation around 18Z and onwards over S-Sardinia. Thunderstorms will then spread north/northeastwards, affecting parts of N-CNTRL Italy during the end of the forecast.

Despite weak mid-level lapse rates beneath the exit area of approaching upper jet, GFS still has a plume of 500-1000 J/kg MLCAPE mainly due to the very moist LL. Needless to say that a lot depends on the final strength of BL moisture recovery, but instability signals, both the placement and the strength, were persistent in the past model runs.

Shear at all levels augments significantly during the day, as intense mid-level streak arrives with DLS increasing to 20-35m/s and 0-3km shear values next to 20m/s. Both, directional and speed shear, are more than adequate for rapid storm organisation with an attendant large hail and severe wind gust risk. The final tornado threat depends on how strong the surface pressure decreases south of Sardinia with attendant backing wind field at lowest 1000m, but very low LCLs also point to a tornado threat. A level-1 was issued as some uncertainties remain but an upgrade may become necessary in upcoming updates.

... S/CNTRL France ...

A well defined surface depression forms just east of NE Spain and drifts slowly northwards during the day. A strengthening pressure gradient ahead of this feature enforces return flow with rapid BL moisture recovery well inland beside the already very moist BL along the coast with dewpoints in excess of 15°C. The main difference to the past events is the rapid cool down of mid-levels as surface depression comes ashore during the midday hours. This trend persists during the afternoon hours with constantly steepening mid-level lapse rates and this should yield robust instability values along the coast, decreasing inland. In addition, forcing is strong as a favorable combination of northward moving mid-level trough axis and approaching left exit region of a mid-level streak overlap temporal and spacious.

20m/s at 850hPa and near 30m/s at 700hPa result in strong shear and LLS values of 20m/s with good directional shear are also present. Environment for surface based convection should be the best along the coastal areas and a level-1 was issued as tornadoes and severe wind gusts are possible. Again the main uncertainty will be the storm mode as inflow is very moist and prolonged period of lift could result in a cluster of storms which lessens chances for discrete thunderstorms.

Further inland, storms should tend to be more elevated although strong moisture advection should help to push the cool/dry BL far northwards. We decided to issue a non-thundery level area far inland as cooling mid-levels assure convective nature of expected precipitation well inland and hence strong wind gusts, isolated large hail and tornado reports are still possible.
Strong WAA up to 3km should keep storms low-topped so lightning strikes should stay more isolated. However, mid-levels continue to cool during the night and GFS develops a nice tongue of elevated instability well inland, so thunderstorm chances (elevated in nature) could increase again during the night and the thunderstorm area may be expanded to the north later-on.

Thunderstorms will also spread eastwards along the coast and offshore, where thermodynamics and kinematics remain favorable for a tornado/severe wind gust and large hail threat.